The present invention relates to an air-fuel ratio control system for an internal combustion engine for vehicles; which controls the air-fuel ratio of the air-fuel mixture to a value approximately equal to the stoichiometric air-fuel ratio at which a three-way catalyst acts most effectively, and more particularly to an air-fuel ratio control system which is capable of preventing an air-fuel ratio adjusting electromagnetic valve from resonating with the vibration of the engine and with the pulsations of induced air.
In a conventional air-fuel ratio control system, the air-fuel ratio of the air-fuel mixture burned in the cylinders of engine is detected as an oxygen concentration of the exhaust gases by means of an O.sub.2 -sensor provided in the exhaust passage of the engine, and a comparator compares the output signal from the O.sub.2 -sensor with a reference value and produces an output representing whether the signal is greater or smaller than the reference value corresponding to the stoichiometric air-fuel ratio. An electromagnetic valve is operated in dependency on the output for regulating the air to be mixed with the mixture to provide the stoichiometric air-fuel ratio.
The output of the comparator is applied to an integrating circuit which produces an output increasing or decreasing in accordance with the integration of the input. The output of the integrating circuit is applied to a comparator where the output is compared with a triangular pulse train applied from a triangular pulse generator for producing square pulses. The duty ratio of the square pulses varies in accordance with the output of the integrating circuit. The square wave pulses are applied to an on-off type electromagnetic valve. Thus, the electromagnetic valve is opened and closed at duty ratios dependent on the applied square pulses for controlling the air to be supplied to the carburetor of the engine to provide the air-fuel mixture at the stoichiometric air-fuel ratio.
The electromagnetic valve is driven at the frequency of the square pulses which corresponds to the frequency of the triangular pulses. It is advantageous for improving the resonance of the air-fuel ratio control system to drive the electromagnetic valve at a high frequency. However, since the high frequency driving of the electromagnetic valve causes a short life time of the valve, the valve is driven at a relatively low constant frequency, in practice.
On the other hand, there is necessarily an operating condition in which vibration of the electromagnetic valve body caused by its own operation coincides with the vibration caused by the engine body and/or with the vibration caused by pulsations of induced air, that is resonance of the electromagnetic valve occurs. When the electromagnetic valve resonates, the air-fuel ratio fluctuates greatly, which will impair the driveability of the vehicle.